If we have 16dm^3 at 120ºC this means we must have 0.5 moles (as 32dm^3 is 1 mole and 32/16=2)
Therefore we find that the Mr we are getting is 46/0.5=92 as (n=m/Mr)
This turns out to be double the Mr of methanol acid (which is 12+16x2+2)
This must mean there are two molecules of methanol acid somehow joined together, given the presence of OH groups in methanol acid, we can deduce this is a case of hydrogen bonding between two formic acid molecules as shown in the mark scheme (where the lone pair of electrons in Oxygen delta- is attracted to the delta+ hydrogen of the other molecule)